Extend `VisionAddOn` Pattern to Qwen2.5VL

[mattjcly]

Currently, the only multi-modal models that have been migrated to the "unified" architecture are Gemma3 and Pixtral:

mlx-engine/mlx_engine/model_kit/model_kit.py

Lines 35 to 38 in ecc2cf4

	VISION_ADD_ON_MAP = {
	"gemma3": Gemma3VisionAddOn,
	"pixtral": PixtralVisionAddOn,
	}

Extending this pattern to Qwen2.5VL/Qwen2VL is desired.

Relevant mlx-vlm components:

• https://github.com/Blaizzy/mlx-vlm/tree/2068970094c78878c77fd78677d1316933562ade/mlx_vlm/models/qwen2_5_vl
• https://github.com/Blaizzy/mlx-vlm/tree/main/mlx_vlm/models/qwen2_vl

Relevant mlx-lm components:

• https://github.com/ml-explore/mlx-lm/blob/77edf17bc0bf7c9313e0b970490db86a4f64bee4/mlx_lm/models/qwen2.py

This will likely look like:

1. Ensure Qwen2.5VL text model architecture is implemented correctly in mlx-lm (including MRoPE, see https://arxiv.org/abs/2502.13923 for details and Apply PR #319 fixes to Qwen 2.5VL position id #349 for mlx-vlm in progress work)
2. Implement Qwen2_5_VLVisionAddOn and wire it in ModelKit
3. Ensure Qwen2.5VL tests in mlx-engine still pass